Bitcoin. You do not need to understand which coin and all that, you just need to consider that it is possible to focus on how you could make money based on what bitcoin has provided.
As long as you have that mindset, then it should be fine, you could of course invest into other coins just like what others do, but that doesn't mean that it will have to be like that, it's just an obvious situation where you could make a big difference when the time comes and could just put aside a small amount to invest into those.
There are so many coins created from time to time. It is so tiring to learn and can be costly if you will consider each of them. It's fine if there are guaranteed return but the problem is there isn't so why can't we just focus on Bitcoin instead? However some old alts are still useful in certain times, like for example now, the Bitcoin fees are huge again. We can use those alts as an alternative for a while but once the situation returns to normal, we can go back in Bitcoin again.
Nothing has ever passed it so far and I bet that nothing else will ever pass it neither.
There is one coin before which value is more than Bitcoin but I don't know what happened to it. Even if the coin is alive, I think Bitcoin is taking the lead now because the value of BTC have improved.
If you do not invest into bitcoin at all, and only focus on alts, that would be a mistake and you should avoid that if possible.
If there are people who only focus on Bitcoin, then there is also people who only like altcoins. There is no problem with that.